This invention relates to a tuning device employing a phase locked loop (PLL) synthesizer for use in a radio receiver.
A digital tuning device employing the PLL synthesizer is extensively used in FM receivers. In a digital tuning device for the PLL synthesizer, if it is necessary to generate a digital code for determining the frequency division number of a programmable frequency divider inserted in the PLL.
For this purpose, a purely electronic method a combination of an up-down counter and a multivibrator, etc. has been proposed. A method using a memory board, such as punched card, for storing binary information and a sensor for reading the binary information on the memory board has been also proposed for this purpose.
However, the purely electronic method is disadvantageous in that it is necessary to provide an upper and lower limit detecting circuit. As a result the manufacturing cost becomes higher than that of the second method using a memory board.
The latter method is superior to the purely electronic method in view of cost considerations as mentioned above. However, the second method has a number of disadvantages which will now be described.
Referring to FIG. 1a and FIG. 1b a digital code storing perforated board 1 has perforations through which light from a light source 2 passes to give a predetermined digital code to photo-transistors 6-1 through 6-N on a photo-transistor holder 4 (FIG. 1(b)). The perforated board 1 may be planar as shown in FIG. 1(a) or cylindrical. That is, the configuration of the perforated board 1 may be selected as desired to satisfy differing requirements such as space, packaging, etc.
When the photo-transistor holder 4 is on line A or B of the perforated board 1 for instance, no difficulty is encountered; that is, the relevant digital code can be properly sensed. However, when the photo-transistor holder is on the line C between the lines A and B, the bits of the digital code may be all at an "L" level or the digital codes on the lines A and B may be erroneously sensed and used as input signals. Thus, the positional relationships between the perforations 3 in the perforated board 1 and the phototransistors 6 may involve problems to be solved.
In order to solve this problem of proper orientation, a method has been proposed in which a click board 5 is arranged below the perforated board 1. A leaf spring 9 abutting against the teeth of the click board 5 is secured to the holder 4 so that the photo-transistors 6 are moved regularly or at predetermined intervals. However, if this technique is utilized in a receiver, it is difficult to obtain the feeling of smooth operation which has been conventionally provided by the combination of a tuning knob and a flywheel. In addition, the method is disadvantageous in that the width of movement of a dial pointer which can be achieved by one discrete operation since the inertia of the flywheel is reduced, and the tuning operation becomes rather difficult.